HYPOTHESIS : The demyelinating brain disease PML arises as the result of a reactivation of a latent/persistent JC virus (JCV) infection following severe immune deficiency. This deficiency leads to altered production of cellular factors regulating JCV expression and/or loss of immune surveillance. The strain of JCV circulating in the population resides in the kidney; it differs from the strain that infects and destroys oligodendrocytes in the brain. This latter JCV variant evolves from the circulating form via deletion and duplication of promoter-enhancer sequences. These rearrangements occur during active replication of the virus in brain, kidney or lymphocytes, with the latter cells also serving as a vehicle to transport virus throughout the body. The following aims are designed to give us a fuller understanding of the pathogenesis of PML, a fatal disease that often afflicts AIDS patients. Completion of this work will support our long-term objective of defining the balance between viral latency and pathogenicity. With this knowledge will come the ability to control and prevent PML. AIM 1. To conduct a functional analysis of the recently discovered JCV proteins T'165, T'136 and T'135 that arise by alternative splicing of the early viral mRNA. Using genetic and biochemical approaches we will investigate the contribution of each T' protein to JCV lytic and transforming behavior. AIM 2. To determine the extent of and molecular basis for JCV latency, persistence, and productive infection in cells derived from normal and diseased individuals. PCR and RT-PCR approaches will be taken to determine whether JCV is latent (DNA present, but unexpressed) or persistent/active (DNA present, and expressed) in various tissues. AIM 3. To investigate the ability of the transcription factors Oct-1 and Tst-1 to influence JCV vs SV4O TAg mediated DNA replication. Using an in vitro DNA replication assay, we will test the prediction that Oct-1 interferes with JCV TAg dependent replication of the SV4O origin. Contributions of the 3 T' proteins to DNA replication will be assessed.